Phloem feeders, such as the psyllid
Diaphorina citri
(
D. citri
), feed on plants by inserting their stylet bundle followed by probing the apoplast before reaching the phloem. The psyllids secrete watery saliva containing various proteins into the phloem, which can act as effectors to facilitate their feeding or modulate host defense responses. Concomitantly, feeding is the main mode of transmitting the
Candidatus
Liberibacter asiaticus (
C
Las) bacteria to the phloem.
C
Las produces several effectors that have been hypothesized to contribute to Huanglongbing (HLB) virulence. Here, we aimed to identify putative effector proteins from both
D. citri
and
C
Las. To achieve this, we used different omics techniques on different tissues and organs from both plants and insects. More specifically, we performed transcriptomics on the heads of healthy and
C
Las-infected
D. citri
and proteomics of artificial diet and of phloem of four different plant species fed on by healthy and
C
Las-infected
D. citri.
Subsequently, we used various criteria and bioinformatics tools to predict putative effectors. This resulted in the identification of four proteins from
D. citri
[ferritin, prisilkin, CG31997-PA, and pterin-4-alpha-carbinolamine dehydratase-like protein (PCBD)] and two Sec-dependent effectors from
C
Las, CLIBASIA_04560 and CLIBASIA_05320, that were used for further functional studies. The expression of these six proteins in
Nicotiana benthamiana
modified the ROS burst triggered by flagellin, indicating that they can indeed function as effector proteins
in planta
. In addition, expression of the psyllid effectors
in planta
significantly reduced the growth of
Pseudomonas syringae
pv.
tabaci
(
Pta
).